In the drilling of oil and gas wells, such drilling takes place through blowout preventers (BOPs) to provide the necessary potential to control any substantial pressure and blowouts. Usually a plurality of ram type blowouts preventers are stacked one above the other and are topped by an annular type blowout preventer. Generally, it has been the custom to provide each of the blowout preventers with upper and lower necks and flanges or clamp hubs for providing the sealed connection between each of the structures of the stack. In both land and offshore drilling the height of the blowout preventer stack directly relates to the size and cost of the drilling rig, i.e., stacks of greater height require larger rigs and thus increase the cost of drilling.
Efforts have been made in the past to provide a blowout preventer stack which is connected by tie rods and does not require the necks and flanges or hubs to make such connection. U.S. Pat. No. 2,749,078 to T. B. Losey is an example of this type of prior art structure. Such structure includes openings through the bodies of the ram type preventers through which the studs extend that tie the individual preventers together. Nuts are threaded onto each end of the studs to tighten the stack. The assembly of such a stack on a drill rig does not save any substantial height in the rig because the studs would, of necessity, have to be lowered through the stack from above and this results in the height requirements for the rig which are substantially the same if not greater as for other prior art stacks.
The tensioning of the tie rods of these prior devices have required tightening of the nuts. It is difficult to control the tension because of the large size of studs and the variation in the coefficient of friction between the nuts and the bodies they engage and between the nuts and the studs. When such prior stacks have been used they are preferably assembled and the studs tensioned before they are transferred to the drilling location. This results in the rig equipment having to handle the complete stack and not just move it from the test stump to its point of installation on the wellhead. Also, modern large bore high pressure BOP stacks, such as 183/4 inch inside diameter and 15,000 pounds per square inch working pressure, are too large and too heavy to be readily transported when assembled.
The J. L. Biach U.S. Pat. No. 3,015,975 discloses a pressure responsive means for tensioning a bolt. In such device the nut is threaded on the bolt and the pressure responsive device is connected to the protruding end of the bolt above the nut. When pressure is applied, the bolt is tensioned. The crank and gear arrangement rotates a wrench within the device to bring the nut into tight engagement with the surface to maintain the tension and after release of pressure the device is removed.
U.S. Pat. No. 4,268,011 and British Pat. No. 1,492,064 of 1977 disclose pressure responsive tensioning devices which include devices to tension studs in pairs.
British Pat. No. 1,507,606 of 1978 and Russian Pat. No. 541,661 disclose pressure responsive devices for tensioning a series of studs with pressure being applied to establish the tension in the studs and then nuts are tightened to maintain such present tension.